It is known that circuit carriers, for example, conventional printed circuit boards, are generally provided before the assembly thereof with printed conductor structures. The printed conductor structures, usually copper and the plated through holes, are provided with a protective layer. The protective layer is to ensure that the soldering points which are to be formed during assembly satisfy electrical and mechanical requirements. These protective layers are thus used to ensure solderability and are often called solderable end surfaces. The solderable end surfaces in current circuit carriers are generally formed from pure tin, because of the current demand for lead-free soldering.
A known drawback of tin surfaces is that they tend, to a greater or lesser extent, to form so-called “whiskers” over the course of storage. For instance, needle-shaped tin monocrystals may become a few micrometers in length and can severely impair the functioning of the circuit carrier. In this case, the whiskers may, on the one hand, cause short-circuits through bridge formation but may also, on the other hand, cause considerable damage when broken off.
The specific cause of whisker formation cannot necessarily be explained, however, it is generally believed that mechanical stresses, due to stress-induced crystallization, may cause the whisker formation. In fact, it may take a seven week storage time at room temperature, for conventional tin surfaces to form whiskers with a length of 30 μm to more than 100 μm.
The changeover required in conjunction with the European guideline “Restriction on Hazardous Substances” from tin/lead to lead-free tin layers is linked with the subject of “whisker” formation. The reason for this is that some of the favored lead-free alternatives tend to whisker formation more than tin/lead solutions do. In addition, the reduction in the printed conductor and connecting portion spacings additionally increases the risk that when whiskers are formed they eventually form an electric short-circuit between adjacent electric printed conductors which can lead to system failure.
Known solutions to these problems aim to avoid or substantially minimize the growth of whiskers for the reliable manufacturing of subassemblies for the automotive industry.
It is known, for example, that by using different types of metals in the circuit carrier and/or the connecting portions, the whisker formation can be reduced.
The drawback in these known solutions is that the circuit carrier and/or the connecting portions have to be changed with regard to their composition, and therefore demand higher costs. In addition, the addition of added metals are problematic in the production of circuit carriers for other reasons.
A problem exists in that whiskers either grow out of the contact regions in which they form, and may cause short-circuits, or else break off and may be present as undesired conductive contaminations. For example, in the case of contacts that are pressed-in, in which the connecting portion of the contact is pressed into a plated through hole of a circuit carrier, a material displacement resembling the track of a snow plough, results in an accumulation of tin in certain regions of the plated through hole.
There is therefore a need to ensure, in the electrical contacting of circuit carriers, that no functional disturbances occur through whisker formation, but without having to carry out changes to the circuit carrier processes or the actual contacts, in the process.